A reflectron is a device incorporated into the flight tube of a  time-of-flight mass spectrometer that operates as an electrostatic mirror. Ions with a range of kinetic energies from the  ionization source traverse the first portion of the flight tube  and then spend different amounts of time in the reflectron.  The net result is that these (reflected) ions all come into focus  at the detector located at the end of the second portion of the  flight tube, leading to higher mass resolution.